Thermisches Verhalten von 1,2-Dipropenylbenzolen

1-cis, 2-cis-Dipropenylbenzene (cis, cis-1) isomerises thermally at 215–235° with 1st order kinetics to give trans, cis-1 and vice versa. At equilibrium 89% trans, cis- and 11% cis, cis-1 are present. It is shown by thermal rearrangement of cis, cis-2′, 2″-d2-1 that the isomerisation is attributable to aromatic [1, 7a]-sigmatropic H-shifts. trans, trans-1 rearranges thermally at 225–245° to yield 2, 3-dimethyl-1, 2-dihydronaphthalene (2). The formation of 2 can be visualized by disrotatory ring closure followed by an aromatic [1, 5s]-sigmatropic H-shift. 2 is also formed when, cis, cis- or trans, cis-1 are heated for 153 h at 225°. Besides 2 a small amount (3%) of 1-ethyl-1, 2-dihydronaphthalene (5) is formed. The rearrangement of trans, trans-1 and trans, trans-2′, 2″-d2-1 shows a secondary isotope effect kH/kD = 0,90.